1. Field of the Invention
This invention relates to voltage converters, especially to DC-DC or switched voltage regulators and to methods and apparatus for control of such regulators.
2. Description of the Related Art
In electronic equipment in general, and particularly for fast moving segments such as portable consumer devices such as mobile phones, MP3 players etc, there is a relentless push to use the latest processor technology to increase the device capability and feature set while reducing power and cost. As the next generation of processors start to become available, they need to operate from lower voltages than previous generation processors so as to allow use of the reduction in process feature size, i.e. W/L, that enables a greater level of integration such that it is beneficial in terms of reduced die size, reduced die cost and reduced power consumption. Typically such processors are powered via a DC-DC voltage converter which provides a regulated voltage to the processor.
These trends result in two design challenges for DC-DC converters to service these applications: one induced by the choice of the value of the low voltage; the other by battery technology lagging behind the change to lower processor supply voltages.                (i) The reduction in processor supply voltage requires a much tighter control—in absolute terms—of the DC-DC converter output voltage under all prevailing processor load and battery conditions. If the control over the processor supply voltage is not sufficient, problems with under or overvoltage may occur: both equally undesirable.        (ii) Since the battery terminal voltage has not dropped appreciably, and the duty cycle of a DC-DC converter is given by the ratio of VOUT/VIN, duty cycles must therefore reduce. This, coupled with the desire for small external components pushes the DC-DC converter to high operating frequencies, resulting in extremely short switch on i.e. conduction, times. The increase in switching speed afforded by the reduction in transistor feature size is not normally available for the power switches since the interface components must be rated to battery voltage.        
Since small conduction periods, i.e. on times, of the power switches are difficult to control, it is becoming increasingly difficult to control the lower processor output voltages with adequate accuracy using traditional Peak Current Mode control methods. Valley Current Mode (VCM) is an alternative method of control of a DC-DC converter which has been proposed. This method of DC-DC loop control controls the input transistor off, i.e. non-conduction, time, rather than the on time. For the low duty cycle required, the non-conduction time is longer than the conduction time, so is therefore easier to control. Also VCM is known to offer an inherently higher bandwidth and an improved transient response.